In the art of papermaking, stock is discharged from a headbox through a slice lip onto a forming wire. On the forming wire, water is drained from the stock so that a web is formed on the wire. The stock is fed to the headbox from an upstream stock header which delivers stock at high pressure to the headbox. When stock is fed to a headbox in a papermaking machine, the stock is commonly fed from the stock header to the headbox in a number of separate conduits leading to the inlet of the headbox and connected to the inlet of the headbox at a number of evenly spaced connection points arranged in a straight row that extends in a cross machine direction. In this manner, the stock supplied to the headbox will be evenly distributed in the cross machine direction in order to ensure that the stock discharged from the headbox is uniformly distributed in the cross machine direction such that the paper web produced by the papermaking machine will have uniform properties, such as basis weight and fiber orientation, across the web width.
However, during the papermaking process, inadequacies due to the construction of the papermaking machine as well as physical phenomena will result in a failure to achieve uniform properties across the web and the paper web produced will have non-uniform properties across its width. In order to compensate for such non-uniformities, it is a common practice to regulate in the cross-machine direction the volume of stock discharged from the headbox. A well-known way of achieving this is to employ in the headbox a slice lip which is locally adjustable. By varying the opening of the slice lip in a cross-machine direction, it is possible to adjust in a cross-machine direction the volume of stock discharged from the headbox. However, this method of regulating the stock flow from the headbox has the disadvantage that when the flow of stock is altered at a point across the slice lip, this change will cause crossflows that will affect both basis weight and fiber orientation. In order to overcome this problem, it has been proposed that the stock fed to the headbox can be selectively diluted in such a way that a diluent is fed into at least some of the conduits through which the stock is fed to the headbox. By regulating the amount of diluent fed into each conduit, it is possible to compensate for basis weight variations in the cross-machine direction. Arrangements for selectively diluting the stock fed to the headbox are disclosed in, e.g., U.S. Pat. No. 5,196,091 (Hergert) and U.S. Pat. No. 4,897,158 (Weisshuhn et al.).
When such an arrangement is used, a flow of stock will meet and mix with a flow of diluent at a meeting point of the respective flows such that the flow of stock and the flow of diluent will combine into a resulting total flow of diluted stock which is the sum of the flows meeting each other, such that Q.sub.tot =Q.sub.s +Q.sub.d where Q.sub.s is the volume flow of stock, Q.sub.d is the volume flow of diluent and Q.sub.tot is the resulting total volume flow of diluted stock downstream of the meeting point.
Since it is to be expected that the stock in each conduit will not be diluted to the same extent, it is of the greatest importance that the feeding of a diluent into the stock will not affect the total volume flow of stock or diluted stock downstream of the meeting point between the respective flows. If the total volume flow of diluted stock reaching the slice lip is not uniform in the cross-machine direction, this will cause crossflows that will affect fiber orientation. Therefore, it has been recognized that when a diluent is fed into the stock on its way to the slice gap, steps must be taken in order to ensure that the total volume flow of diluted stock in a stock conduit remains constant regardless of the size of the volume flow of the diluent which is fed into the flow of stock, such that downstream of the point where a diluent is fed into the stock, the volume flow of diluted stock will be the same in each stock conduit.
A solution to the above-mentioned problem is disclosed in U.S. Pat. No. 5,316,383 (Begemann et al.). According to this document, a first volume flow of liquid (e.g., stock) coming through a first inlet line is met by and mixed with a second volume flow of liquid (e.g., a diluent, such as water or diluted stock) coming through a second inlet line. The two liquids mix with each other at a meeting point and form a resulting total volume flow of mixed liquid. The first inlet line is disposed at a mixing angle relative to the second inlet line and the mixing angle is chosen such that the resulting total volume flow of mixed liquid remains constant. According to this document, the solution to the above-mentioned problem thus lies in the proper selection of the mixing angle. The preferred mixing angle for one embodiment is specified as 80.degree.. This document also describes experiments with different mixing angles which demonstrate that, at a mixing angle of 90.degree., an increase of the second volume flow will result in a decrease of the total volume flow of mixed liquid.
A different solution to the problem of achieving a constant volume flow of mixed liquid is disclosed in the above-mentioned '091 patent to Hergert. According to this document, a headbox apparatus is provided with a tapered inlet, or stock header, for the flow therethrough of stock. The upstream end of a tube bank having a plurality of tubes for the flow therethrough of stock is connected to the tapered inlet such that stock flows through the tapered inlet and from the inlet through the upstream end of the tube bank to a downstream end of the tube bank. A plurality of supply conduits for a diluent, such as fresh water or white water, are connected to the upstream end of the tube bank. Each supply conduit has a termination disposed closely adjacent and upstream relative to an upstream tube in the tube bank. In FIG. 6 of the '091 patent, the termination of the supply conduits is shown as being located in a wall of the tapered inlet, such that a supply conduit will feed a diluent directly into the stock at a location upstream of the upstream end of a tube in the tube bank. According to the '091 patent, when a diluent, such as water, flows through the termination and into the stock, the stock flowing through a tube adjacent the termination will be diluted, but the flow rate through the tube will not be changed. In this case, the constancy of the flow rate is due to the fact that the diluent is fed into the stock upstream of the tube.
Each of the above-mentioned solutions offers a possibility to dilute stock flowing through a tube without affecting the total volume flow of the stock through the tube. However, the known solutions are not always satisfactory. For example, the solution according to the '091 patent entails a potential risk that the diluent can not be fed into the stock with complete accuracy since the diluent is fed directly into the tapered inlet, or stock header, at a point upstream of the tube ends where it could possibly fail to reach the correct tube end. Furthermore, there is a possibility that the feeding of a diluent directly into the stock header might cause pressure variations in the header that might affect the flow in the stock conduits. The solution according to the '383 patent requires a specially designed connection between the inlet lines of the respective volume flows since the flows must meet each other at the correct angle and the required slanted connection is, from a manufacturing point of view, not the simplest design.
Therefore, there is a need for an easily applied solution to the problem of feeding a volume flow of a diluent into a volume flow of stock such that the flow of stock and the flow of diluent will form a resulting total volume flow of diluted stock where changes in the volume flow of the diluent will not affect the resulting total volume flow of diluted stock. There is also a need for an arrangement for diluting stock which is simple to manufacture. In addition, there is a need for an arrangement for diluting stock which ensures a good mixing of the stock and the diluent. There is furthermore a need for an arrangement for feeding stock to a headbox which allows an easy installation under various conditions.